Power wrench

ABSTRACT

A power screwdriver has a drive unit and an input circuit for inputting screw size, material quality, pitch, and clamping length of a screw. An evaluation circuit is provided for determining a nominal prestress force based on input screw size, material quality, and clamping length, for determining a nominal torque based on the nominal prestress force and the pitch, and for determining the screw clamping angle resulting from the nominal prestress force and the pitch. A sensing device for sensing the actual torque is provided, and a further sensing device for sensing the actual screw clamping angle is present. A control and switch-off device controls further rotation of the drive unit when the actual torque coincides with the nominal torque and switches off the drive unit when the actual screw clamping angle coincides with the nominal screw clamping angle. The nominal torque, based on which the additional rotation o f the drive unit is controlled until the nominal screw clamping angle is reached, is set to a value which is smaller than the torque corresponding to the nominal pretension force. An additional rotation angle of the drive unit, for reaching the nominal screw clamping angle corresponding to the nominal prestress force, is determined based on the difference between the nominal screw clamping angle and the adjusted screw clamping angle corresponding to the preset nominal torque and is used for switching off the drive unit.

BACKGROUND OF THE PRIOR ART

The invention relates to a power screwdriver with a drive unit, asensing device for the actual torque as well as optionally for theactual screw clamping angle, and an electronic control of therevolutions per minute and/or the torque and/or the screw clampingangle.

In German patent 43 07 131 a power screwdriver is disclosed having adrive unit having a with planetary gear downstream thereof, a torquedetection at the drive shaft, an evaluation electronic circuit, and aswitch-off device for the drive unit, a sensing device for detecting therotational position of the drive unit whereby the switch-off device hasa adjustable preset device for a certain torque determined by the torquedetection device, and additionally an adjustable preset device, for acertain follower path determined by the rotational position detectiondevice and wherein switching off of the drive unit after reaching thepreset torque is carried out followed by a further rotation by anadjusted follower path.

Before performing a tightening operation, the required torque as well asoptionally the required follower path are preset by the correspondingadjusting elements. Subsequently, the power screwdriver is placed ontothe screw element and a support leg is moved against a fixed abutment.After starting the drive unit, the tightening operation is performed andthe screw element is tightened so that the torque increases. Themeasuring-technologically determined torque value is compared to thetorque value that is preset, and, when both values are identical, aninternal switching signal is provided for the follower control. Based onthe actual rotational position the drive unit is then rotated further bythe preset follower path. After completion of the preset follower path,the switching signal is released by the switch-off device for switchingoff the drive unit and thus terminating the threading operation.

In the known power screwdriver the operator must determine with respectto the screw size and the material quality the required nominal torquewith the aid of tables and must input the data into the electronic inputdevice. The follower path must also be determined by the operator andoptionally entered into the electronic input device. The function of thefollower path and how to determine it is not disclosed in the Germanpatent 43 07 131. Also, the measuring-technologically detected torquevalue depends greatly on external circumstances such as thread frictionand is no precise measure for the actually reached screw pretension.

The invention has the primary object to simplify the operation of apower screwdriver of the aforementioned kind and to eliminateoperational errors, without requiring special knowledge by the operator.Furthermore, damage to the screws to be tightened and to the powerscrewdriver are to be prevented, the precision with which the tighteningactions of the screws is effected is to be increased, the effects of therotational energy of the drive unit during switch-off onto thetightening of the screw is to be reduced or eliminated, and a safetransmission of the measuring and control signals is to be ensured.

SUMMARY OF THE INVENTION

Based on this problem, as a simplification of operation and for avoidingoperational errors of the power screwdriver, it is suggested that theelectronic input device is inventively designed for inputting the screwsize, the pitch, and the material quality and that the electronicevaluation device determines a nominal torque based on the input screwsize, the pitch, and the material quality, whereby the actual torque isdetermined by a sensing device and a switch-off effects the switchingoff of the drive unit when the actual torque and nominal torquecoincide.

The operator thus no longer must determine, based on tables, the nominaltorque for a certain screw size and pitch with a certain materialquality and must no longer input such data into the electronic inputdevice. Instead it is sufficient to input the screw size, the pitch, andthe material quality into the electronic input device so that the thusresulting nominal torque is determined by the electronic evaluationdevice and serves for controlling the power screwdriver. Erroneousadjustments, which result in damage to the screws to be tightened andthe power screwdriver, are prevented because no special knowledge isrequired by the operator.

Based on the aforementioned problem it is further suggested forsimplifying operation and for increasing the precision during tighteningof the screw by the power screwdriver, that the electronic input devicefor is desired inputting the screw size of the material quality, thepitch, and the clamping length, that a nominal pretension force aredetermined based on the input data by the electronic device, a nominaltorque and screw clamping angle whereby a sensing device determines theactual torque and a sensing device detects the actual screw clampingangle and a control and switching device is provided witch effectsfurther rotation of the drive unit upon coinciding of the actual torqueand the nominal torque and switching off of the drive unit uponcoinciding of the actual screw clamping angle and the nominal screwclamping angle.

This inventive control device for the power screwdriver is based on theidea that the precise adjustment of the screw prestress force may resultfrom monitoring the screw expansion because the screw expansion,according to Hook's Law, is a linear function of the prestress force,the cross-section, the clamping length, and the module of elasticity.This screw expansion is directly proportional to the screw clampingangle for geometric reasons, but the zero value of the expansion cannotbe directly determined from the screw rotation because the screw isfirst rotated with minimal torque until the parts to be clamped contactone another, the point of connection, and only thereafter a linearincrease of the prestress force with screw expansion will occur. Thenominal prestress force can be determined by computation of the inputscrew size, the material quality, and the clamping length, and, based onthis, in connection with the input pitch, the nominal torque and theresulting screw clamping angle can be determined. It is thus sufficientto first measure the actual torque to a predetermined level and to thenallow the control and switch-off device to act on the power screwdriversuch that beginning at this level the addition rotation of the driveunit by a certain angle, which results in the desired nominal prestress,is carried out.

In this inventive solution the operation is also greatly simplified andoperational errors are prevented because all data are input into theelectronic input device and the electronic evaluation device willautomatically determine the control data based on the input.

Preferably, the nominal torque which is the starting point for furtherrotation of the drive unit until the nominal screw clamping angle hasbeen reached, is set to a value which is smaller then the torquecorresponding to the nominal prestress force and the additional angle ofrotation of the drive unit until the nominal screw clamping anglecorresponding to the prestess force has been reached is determined basedon the difference between the nominal screw clamping angle and the forthe screw clamping angle corresponding to the input nominal torque andis used for switching off the drive unit.

It is especially preferred that the nomiani torque is adjusted to 10% to20% of the torque corresponding to the nominal prestess force and thatthe further rotational angle is adjusted to 90% to 80% of the nominalscrew clamping angle corresponding to the nominal prestress force.

It has been shown in practice that after reaching 10% to 20% of thetorque corresponding to the nominal prestress force, the linear range ofthe screw expansion has been reached so that the further additionalrotational angle can be adjusted to 90% to 80% of the nominal screwclamping angle corresponding to the nominal prestress force, and thisresults in a very precise tightening of the screw with a desiredprestress force because inexactness during the starting phase oftightening of the screw will have only a small impact onto the finalprestress force.

Based on the aforementioned problem, a power screwdriver is suggested inwhich the effect of the rotational energy of the rotating parts onto thescrew prestress force is reduced or eliminated and which comprises anelectronic input device for inputting a nominal torque or for inputtingthe screw size, the pitch, and the material quality or for inputting thescrew size, the pitch, the material quality, and the clamping length,optionally comprises an electronic evaluation device for determining anominal torque based on the input screw size, the pitch, and thematerial quality and/or for determining a nominal prestress force andthe values to be determined therefrom, such as nominal torque and/ornominal screw clamping angle, further comprises a sensing device for theactual torque and/or the actual screw clamping angle, and furthercomprises a control and switch-off device for a continuous reduction ofthe rpm of the drive unit with decreasing difference between the nominaltorque and the actual torque or between the nominal screw clamping angleand the actual screw clamping angle and for switching of the drive unitupon coinciding of the actual torque and the nominal torque or theactual screw clamping angle and the nominal screw clamping angle. Thereduction of rpm can be started immediately with increase of the torque,however, it is also possible to perform the threading operation firstwith high rpm and, only after reaching a predetermined moment, to startreducing the rpm. This results in a shortening of the threadingoperation.

The control and switching device for controlling the rpm of the driveunit effects that the rotational energy of the power screwdriver uponapproaching the nominal value is more and more reduced and therotational energy, at the moment of switching off, is identical orsubstantially identical to zero and has no effect on the actualprestress force.

Such a rpm control can be performed in any power screwdriver withelectronic torque limitation or also in a power screwdriver with acontrol and switch-off device having an electronic screw clamping anglelimitation. Especially advantageous is the use of the inventive rpmcontrol in connection with the inventively embodied electronic inputdevice for inputting the screw size, the pitch, and the material qualityand the electronic input device for inputting the screw size, thematerial quality, the pitch, and the clamping length.

It may also be advantageous to design the power screwdriver of theaforementioned kind with a sensing device for the actual torque suchthat the actual torque of a already tightened screw is measured by aslight further rotation and the electronic evaluation device thendetermines based thereon the nominal torque and/or a nominal screwclamping angle for a further tightening of the screw to a value thatmatches the input data.

With respect to space conditions within the housing of the powerscrewdriver, it is advantageous when at least the control and switch-offdevice and optionally also the electronic evaluation device areintegrated into the drive unit, while the input circuit and optionallythe evaluation circuit can be embodied as independent control devicesthat act onto the control and switch-off or onto the evaluation circuitwithout cable connection with bi-directional or uni-directionaltransmission of data. For this purpose, the remote transmission can beembodied as a unidirectional or bi-directional infrared path and acorresponding infrared signal receiver can be arranged at the driveunit.

Especially when the control and switch-off device is integrated into thedrive unit, while the input circuit and the evaluation circuit areembodied as independent control devices, they can be combined in aportable computer which has a bi-directional IrDA interface withinfrared transmission while a corresponding interface at the drive unitis provided.

Uni-directional infrared data transmission devices are known as infraredremote controls for T.V. sets. They allow a uni-directional datatransmission across distances of up to 36 meters under optimalconditions. With these uni-directional infrared data transmissionsystems the stand by current of the sender is substantially equal tozero while during operation the current is only of a size of a fewmilliampere. The receiver also requires only a current of a fewmilliampere, i.e., it is in a range which is possible for portable powerscrewdrivers with conventional current supplies.

Portable computers are often already equipped with IrDA interface. Theyare in the form of standardized bi-directional interfaces with infraredtransmission. A respective interface is arranged at the powerscrewdriver and this IrDA component has a minimal current consumption ofa few microampere in the stand by position and of a few milliampere inthe activated state.

Advantageous for all infrared transmission systems is the optimizedgalvanic separation, which is a result of its operating principle, aswell as the minimal cost for standard components. Furthermore,interferences result by emission of disturbing signals or interferenceby signals. When employing standardized IrDA components, for securingthe transmitted data control bits can be sent. Furthermore, a portablecomputer which is connected to the power screwdriver by a bi-directionalinfrared interface, is suitable to evaluate and document a large amountof data and to thus statistically evaluate a large number of threadingprocesses.

When the input circuit has a display, it can be designed such that thedisplay shows the input values, the resulting nominal values, and/or theactual torque course and/or the course of the actual screw rotationangle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in the following with the embodimentshown in the drawing. The drawing shows:

FIG. 1 a schematic representation of a power screwdriver and theelectronic control elements;

FIG. 2 a graphic representation of the course of the torque M, theprestress force F, and the rpm end as a function of the screw clampingangle.

DESCRIPTION OF PREFERRED EMBODIMENTS

In general, the reference 1 indicates the power screwdriver having adrive unit 2 which is represented as an electric drive unit which,however, can also be operated pneumatically or hydraulically. Aplanetary gear system 3 is connected to the drive unit 2 which, as isshown, has an angular drive but can also extend coaxially to the mainrotational axis of the drive unit. The planetary gear system 3 providesa reduction of the rpm of the drive unit 2 and increases the achievabletorque.

At a neck of the planetary system 3 in the area of the square outputshaft 5 of the planetary gear system 3, a support leg 4 is stationarilyconnected which, upon tightening of the screw, is moved against a fixedabutment and in this manner receives the reaction moment. The squareshaft 5 is designed for receiving a socket wrench.

The drive unit 2 comprises an input circuit 6 which has a keyboard 7and, optionally, a display 8 for showing the input data. With this inputcircuit 6, the screw size, material quality, the pitch, the clampinglength can be input and also displayed on display 8. These values areguided into the evaluation circuit 9 which determines, based on theinput screw size and the material quality, nominal torque or, based onthe screw size, the material quality, and the clamping length, nominalprestress force and based thereon, when taking into consideration thepitch, a resulting nominal torque and, finally, based on the nominalprestress force and the pitch, a resulting screw clamping angle. Thesedetermined nominal values and/or the current actual torque coarse and/orthe current actual screw rotational angle can be displayed on thedisplay 8.

The planetary gear system 3 cooperates with a sensing device 10 for theactual torque and a sensing device 11 for the actual screw clampingangle. The sensing devices 10, 11 can be embodied as a conventionalcontactless sensing device as is, for example, known from German patent43 07 131. The measured values of the sensing device 10, 11 are guidedto the control and switch-off device 12 which acts on the drive unit 2.

Preferably, the sensing devices 10, 11 and the control and switchingdevice 12 and optionally the evaluation circuit 9 are integrated intothe drive unit 2, while the input 6 circuit and optionally theevaluation circuit 9 are embodied independent of the power screwdriver 1as a control device and are connected to the power screwdriver 1 by acable or, in an especially preferred embodiment, without cable by remotetransmission of data into an electronic circuit integrated into thepower screwdriver 1. When an infrared remote control is used, the datainput into the input circuit 6 or the control data that have beenprocessed by the evaluation circuit 9 can be transmitted without cableto the power screwdriver 1. When the input circuit 6 and the evaluationcircuit 9 are combined in a portable computer and when this computer isprovided with a so-called IrDA interface 13 as a remote transmissiondevice, the infrared signals can be transmitted by this interface onto amatching interface 14 at the power screwdriver 1 which is incommunication with the control and switch-off device 12. This IrDAinterface 13 and the corresponding interface 14 are embodied asstandardized bi-directional interfaces with infrared transmission fordistances of u J) to one meter and transmission rates of more than 1MBit/S. These IrDA components 13, 14 have in standby mode a minimalcurrent consumption of a few micoampere and in the activated stage of afew milliampere which is in a range that is feasible for currentsupplies of transportable power screwdrivers. Especially advantageous inregard to these infrared transmission systems is the optimal galvanizedseparation of the input circuit 6 and the evaluation circuit 9 from thepower screwdriver 1, which is a result of the operating principle, andthe minimal costs by using the standardized components. Thesestandardized components have a minimal size so that they can beintegrated without problems into the housing of the power screwdriver 1.

When only the input circuit 6 is independent of the power screwdriver 1,a bi-directional data transmission is not required so that in this caseit is also possible to employ infrared remote controls known from T.V.sets for a unidirectional remote transmission of data. This infraredremote control have a minimal standby current, while the currentconsumption is a few milliampere when active.

When the inventive power screwdriver 1 operates only with torquelimitation, only the screw size and the material quality must be inputinto the input circuit 6 so that the evaluation circuit 9 determines thecorresponding nominal torque. During tightening of the screw the sensingdevice 10 measures the actual torque and the control and switch-offdevice 12 switches off the drive unit 2 once the actual torquecorresponds to the nominal torque.

When a highly precise tightening of the screw to a nominal prestressforce is required, which can be determined based on the screw size, thematerial quality, and the clamping length and to which the screwexpansion is proportional, the input circuit 6 is used to input thescrew size, the material quality, the pitch, and the clamping length sothat the evaluation circuit 9 will determine a nominal prestress force,a nominal torque, and a nominal screw clamping angle.

Upon tightening the screw, the sensing device 10 will measure the actualtorque and the sensing device 11 will measure the actual screw clampingangle, and these values are supplied to the control and switch offdevice 12. The control and switch-off device 12 is designed such that iteffects the additional rotation of the drive unit by a certain angle isaffected upon coinciding of the actual torque with the nominal torqueand switches off the drive unit 2 when the actual screw clamping anglecoincides with the nominal screw clamping angle. The nominal torquebased on which the additional rotation of the drive until 2 uponreaching of the nominal screw clamping angle is controlled, is smallerthan the torque corresponding to the nominal prestress force, preferablyit corresponds to 10% to 20% of the torque corresponding to the nominalprestress force, while the additional rotation angle is then 90% to 80%of the nominal screw clamping angle corresponding to the nominalprestress force.

FIG. 2 shows that the torque M when first turning the screw untilreaching the connecting point, is first at a low value and, only aftersurpassing 10% to 20% of the torque corresponding to the nominalprestress force, will it increase linearly with the screw clampingangle. Accordingly, the initial point for additional rotation angle a at10% to 20% of the torque corresponding to the nominal prestress forceand this further rotation angle a is correspondingly 20% to 80% of thenominal screw clamping angle corresponding to the nominal prestressforce which clamping angle can be precisely monitored.

The rpm in the drive unit 2 can be controlled inventively such that itwill be reduced continuously with decreasing difference between thenominal torque and the actual torque or between the nominal screwclamping angle and the actual screw clamping angle and such that, uponreaching the nominal torque or the nominal screw clamping angle, willreach 0, i.e., the drive unit is switched off. This reduction of the rpmof the drive unit 2, as represented, can be linear, but it is alsopossible to provide a non linear course. It is especially possible toperform the tightening first at high rpm and, only after reaching apreset torque to reduce the rpm. This results in a shortening of thetightening process. The rpm control is shown in FIG. 2 in dashed lines.

With the inventive power screwdriver the operation is considerablyfacilitated by eliminating error sources, the screw prestress force canbe achieved more easily and the limitation of the rotational energy ofthe drive unit as a function of the already dispensed moment of thescrew tightening is limited so that a gear and material protection aswell as a substantially independence of the switch-off moment from thetorque course over time of the screw tightening results.

Furthermore, it is possible to design the sensing device 10 for theactual torque at the inventive power screwdriver 1 such that it willmeasure the actual torque of an already tightened screw by furtherrotation and that the evaluation circuit 9 will determine based thereonthe nominal torque and/or the nominal screw clamping angle for furthertightening of the screw to a value that is determined based on the inputdata.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What is claimed is:
 1. A power screwdriver (1) comprising:a drive unit(2), an input circuit (6) for inputting a screw size, a materialquality, a pitch, and a clamping length of a screw; an evaluationcircuit (9) for determining a nominal prestress force based on inputscrew size, material quality, and clamping length, for determining anominal torque based on the nominal prestress force and the pitch, andfor determining the screw clamping angle resulting from the nominalprestress force and the pitch, a sensing device (10) for sensing theactual torque, a sensing device (11) for sensing the actual screwclamping angle, a control and switch-off device (12) for controllingfurther rotation of said drive unit (2) when the actual torque coincideswith the nominal torque and for switching off said drive unit (2) whenthe actual screw clamping angle coincides with the nominal screwclamping angle; wherein the nominal torque, based on which theadditional rotation of said drive unit (2) is controlled until thenominal screw clamping angle is reached, is set to a value which issmaller than the torque corresponding to the nominal pretension force;and wherein an additional rotation angle of said drive unit (2), forreaching the nominal screw clamping angle corresponding to the nominalprestress force, is determined based on the difference between thenominal screw clamping angle and the adjusted screw clamping anglecorresponding to the preset nominal torque and is used for switching offsaid drive unit (2).
 2. A power screwdriver according to claim 1,wherein the preset nominal torque corresponds to 10% to 20% of thetorque corresponding to the nominal prestress force and the additionalrotation angle is 90% to 80% of the nominal screw clamping anglecorresponding to the nominal prestress force.
 3. A power screwdriveraccording to claim 1, wherein said input circuit (6) is adapted toreceive as additionally input a nominal torque, wherein said control andswitch-off device (12) continuously reduces the rpm of said drive unit(2) with decreasing difference between the nominal torque and the actualtorque and switches off said drive unit (2) when the actual torquecoincides with the nominal torque.
 4. A power screwdriver according toclaim 1, wherein said evaluation circuit (9) determines a nominal torquebased on input screw size, pitch, and material quality, wherein saidcontrol and switch-off device (12) continuously reduces the rpm of saiddrive unit (2) with decreasing difference between the nominal torque andthe actual torque and switches off said drive unit (2) when the actualtorque coincides with the nominal torque.
 5. A power screwdriveraccording to claim 1, wherein said evaluation circuit (9) determines anominal screw clamping angle based on screw size, pitch, materialquality and clamping length, wherein said control and switch-off device(12) continuously reduces the rpm of said drive unit (2) with decreasingdifference between the nominal screw clamping angle and the actual screwclamping angle and switches off said drive unit (2) when the actualscrew clamping angle coincides with the nominal screw clamping angle. 6.A power screwdriver according to claim 1, wherein said sensing device(10, 11), for determining the nominal torque, measures the actual torqueof an already tightened screw by a slight further rotation, wherein saidevaluation circuit (9) determines, based on the actual torque of thealready tightened screw, a nominal torque and/or a nominal screwclamping angle for further tightening of the already tightened screw toa value based on the input data.